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Biogas

Biogas, or methane, is a clean-burning, "green" fuel used for heating and cooking, transport and power generation  and you can make it yourself.

The "natural gas" piped to millions of consumers in North America, Europe and elsewhere is also methane, though it's purer than biogas. Chemically they're the same, but that's where the resemblance ends.

Natural gas is indeed natural, in the same way that petroleum is natural, and, like petroleum, it's a fossil fuel  it's heavily processed, its production depends on fossil fuel inputs every step of the way, and it contributes to global warming.

Like carbon dioxide, methane is a greenhouse gas  in fact it's 23 times worse than carbon dioxide. But, also like carbon dioxide, the methane produced in the natural cycle of growth and decay is climate-neutral. For a detailed explanation of this, see "Greenhouse gases and global warming".
http://journeytoforever.org/biodiesel.html#greenhouse
Making and using biogas is part of the natural cycle, and it doesn't contribute to global warming.

Landfills and tips that leak methane are a major contributor to human-caused global warming.

But you won't be releasing any biogas into the atmosphere, and when you burn it, the only emissions are carbon dioxide and water vapour.

Methane is also called marsh-gas  it's what will-o'-the-wisps are made of, the ghostly lights sometimes seen flickering over marshes at night, the source of much mediaeval fear and superstition, and not without cause. Before they drained their swamps, the English were prone to malaria (previously called "the ague", often mentioned in Shakespeare), and to a disease called impaludism, defined in The Lancet 120 years ago as the general state of morbidity afflicting people who lived too close to a bog, characterised by frequent fevers, enlargement of the spleen and other ailments.

Just thought I'd tell you that. Anyway, you won't get impaludism from making biogas because it all happens inside a closed digester.

Some facts

Biogas usually contains about 55-65% methane, 30-35% carbon dioxide, and some hydrogen, nitrogen and other impurities. Its heating value is around 600 BTU per cubic foot (21 BTU per litre).

Natural gas contains about 80% methane, with a heating value of around 1000 BTU per cubic foot (35 BTU per litre).

Filtering biogas, or "scrubbing" it, can remove the carbon dioxide and the other impurities, raising the BTU.

There are two basic types of digester, batch digesters and continuous digesters. Batch digesters are filled with a mixture of organic wastes and water (slurry) and sealed, and emptied again when they stop producing gas. Continuous-load digesters are fed a daily load of slurry, with gas and digestion wastes produced continuously.

Biogas digestion works best at 25 to 35 deg C, 77-95 deg F.

One pound (0.45 kg) of cow manure can produce about one cubic foot of gas (28 litres) at around 28 deg C (82 deg F)  enough to cook a day's meals for 4-6 people in India.

About 1.7 cubic metres of biogas (60 cubic feet) equals one litre of gasoline (1/4 gallon). The manure produced by one cow in one year can produce enough methane to replace more than 200 litres of gasoline (53 gallons).

Myths

Apart from the gas, digesters also produce waste sludge and supernatant, the spent liquid of the original slurry. The waste is rich in plant nutrients, and it's often touted as a great organic fertiliser. Many of the resources listed below claim that.

However, the people who design biogas digestion systems are nearly always engineers, not biologists, and they tend to think chemicals are chemicals, as indeed they are, and these particular chemicals are indeed both nutrients and organic. But that doesn't make it a fertiliser  in fact it kills earthworms and wrecks the soil micro-life, which is the basis of soil fertility. This is what biologists say about it:

Biogas digestion is an anaerobic process (no oxygen involved), unlike composting, which is aerobic (with oxygen). Compost gets hot, up to 60 deg C or more (140 deg F), biogas digesters don't get hot. Anaerobic digestion produces volatile fatty acids and volatile organic acids, both of which are phyto-toxins  plant poisons. Not what you want in your soil.

Add the sludge and supernatant to your compost pile. Biogas digestion makes the best sense when it's coupled with hot aerobic composting, then nothing will be wasted. Also, the heat in an active compost pile can be harnessed to produce a hot water supply, which means you can use the compost to keep the digester at working temperature during cold weather.

Another myth is that the digestion process kills off the pathogens in the manure (cow, pig, poultry and human manure are common feedstock for biogas digesters). Hot composting kills off pathogens reliably, when it's done right, but biogas digestion doesn't kill the pathogens in the manure.

"Indian biogas plants have short detention times. These are unlikely to destroy intestinal parasites, which are widely prevalent in rural areas of India. As a result, if the biogas sludge were used as a fertilizer, it would likely increase the spread of intestinal diseases."  From "Community Biogas Plants Supply Rural Energy and Water", UNDP

And:

"Work I did and studied in the 1970s with 'dungas' production (anaerobic digestion) in South Africa and India showed that the drawback was the high concentration of pathogens in the resultant slurry. ... It's the pathogen problem that caused me to abandon this line of research as a viable source of alternative energy production and return to aerobic composting."  Walker Bennett, Organic Gardening Discussion List, 15 Oct 1999.

This isn't a problem as long as you're aware of it. Adding the sludge and supernatant to the compost pile will kill all pathogens.

If you're not in a position to make compost, flush the sludge and waste liquid down the toilet  let your municipal wastewater treatment facility handle it for you, that's what they do all the time.

Buts

Producing biogas might not be quite as simple as it sounds.

In a nutshell: "Anaerobic bacteria are sensitive to their environment and require ideal conditions in order to maximize the efficiency of the digestion process. An ideal environment for the bacteria is warm (approximately 95 deg F), oxygen-free, has a pH between 6.6 and 7.6, and contains a mixture of carbon dioxide and methane, as well as a consistent supply of organic matter to 'feed' upon."

"Most opinions indicate that anaerobic fermentation is very sensitive to operate and difficult to control."  "A Biogasification System at a Dairy", Proceedings of the Oklahoma Academy of Science, 56: 18-23 (1976), M. R. Brulé and S. S. Sofer

Don't be put off though, you can do it. Study the resources below carefully, decide what's best for you in your particular circumstances, and go ahead.

Biogas as motor fuel

Gas - 10 Cents per GALLON - YES!!!  Peter H. Weis, Vancouver.
"You can run your car, if you wish, on fuel which would cost you about 2.5 cents per litre - or about 10 cents per gallon - and get 5 miles per gallon more than you do now. And there will be much cleaner exhaust  plus a very substantial reduction of carbon dioxide emissions as well, which will be a considerable contribution to the reduction of the greenhouse effect."http://www.truehealth.org/methane.html

Put a chicken in your tank -- Eccentric British inventor Harold Bate found a way of converting chicken droppings to biogas and running his car on it. He claimed chicken power would run a car faster, cleaner, and better than gasoline. Bate said he'd driven his 1953 Hillman at speeds up to 75 mph without the use of gasoline.

Nepal Biogas Plant -- Construction Manual. Construction Manual for GGC 2047 Model Biogas Plant. With Dutch and German support, Nepal's Biogas Support Programme has built 95,400 biogas plants in 10 years, with potential for half a million more. These are fixed dome biogas plants, designed in Nepal. Sizes are household-scale from 4 to 20 cubic metres. The feedstock is cattle dung and water (but other feedstocks will work just as well). For instance, the 4-cubic-metre plant requires input from 2-3 cattle, the 10-cubic-metre plant needs 6-9 cattle. This manual includes full construction details, plans and data. With thanks to Olivier Morf.

Jean Pain: France's King of Green Gold -- In the 1980s Frenchman Jean Pain built a home-made power plant that he claimed supplied 100% of his energy needs. The core of the system was a 50-ton compost mound, three metres high and six across, made of pulverized tree limbs and underbrush. Buried inside the compost was a 4-cubic-metre sealed steel tank 3/4-full of the same compost, producing methane -- bio-gas. Tubes connected the tank to a pile of 24 truck-tyre inner tubes, the gas reservoir. Pain said he used the gas to cook all the food, fuel a truck and produce electricity, via a methane-fuelled internal combustion engine that turned a generator. The truck ran off two gas bottles on the roof. Another tube ran from a well and into the heap, with 200 metres of tubing wound round the tank; Pain said the water emerged at 60 deg C at 4 litres a minute, which was enough for central heating, and for the bathroom and the kitchen. The compost heap continued fermenting for nearly 18 months, and then yielded 50 tons of natural fertilizer. (With thanks to Ramjee Swaminathan.)